1. Field of the Invention
The present invention relates generally to variable frequency oscillating circuits and, more particularly, to a variable frequency oscillating circuit embodied as a semiconductor integrated circuit and not requiring a variable capacitance diode.
2. Description of the Prior Art
In general, variable frequency oscillating circuits whose frequency of oscillation can be controlled by an input voltage or input current are typically referred to as voltage controlled oscillators (VCO) or current controlled oscillators (CCO). Such VCO's and CCO's are then further roughly classified either as being of the inductance and capacitor (LC) oscillatory kind or as being of the multivibrator oscillatory kind.
Generally, a variable frequency oscillating circuit of the LC kind employs a variable capacitance diode as the resonance element, and a control voltage is supplied thereto and varied to control the oscillatory frequency of the circuit. In the case of the variable frequency oscillating circuit of the multivibrator kind, the frequency of oscillation is varied by controlling the charge/discharge current of the multivibrator feedback capacitor, that is, by controlling the triggering threshold level.
Nevertheless, if the variable frequency oscillating circuit is of the LC kind and its variable capacitance diode is an ultra-stage junction type one, the cost of manufacturing such variable frequency oscillating circuit increases substantially compared with a variable frequency oscillating circuit using an ordinary variable capacitance diode. Moreover, a variable capacitance diode formed as an ultra-stage junction is not easy to fabricate along with the other necessary circuit elements on the same semiconductor, integrated circuit chip and, accordingly, such variable frequency oscillating circuit is not suitable for forming as an integrated circuit (IC). Furthermore, when a diode of the ordinary PN junction kind is employed, its variable capacitance range is so narrow that the range of frequency variation that is possible in the variable frequency oscillator is also similarly restricted.
The variable frequency oscillating circuit of the multivibrator kind also has drawbacks because of oscillatory instability, which results in only a VCO or a CCO having poor carrier-to-noise (C/N) ratio being able to be practically constructed. Accordingly, a variable frequency oscillating circuit of the multivibrator kind cannot provide the same or better efficiency as a variable frequency oscillator of the LC kind.